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Purpose Radioisotopes such as 75 Se, 169 Yb, and 153 Gd have photon energy spectra and half-lives that make them excellent candidates as alternatives to 192 Ir for high-dose-rate brachytherapy. The aim of the present study was to evaluate the relative biological effectiveness (RBE) of current (192 Ir, 125 I, 103 Pd) and alternative (75 Se, 169 Yb, 153 Gd) brachytherapy radionuclides using Monte Carlo simulations of lineal energy distributions. Methods and Materials Brachytherapy sources (microSelectron v2 [192 Ir, 75 Se, 169 Yb, 153 Gd], SelectSeed [125 I], and TheraSeed [103 Pd]) were placed in the center of a spherical water phantom with a radius of 40 cm using the Geant4 Monte Carlo simulation toolkit. The kinetic energy of all primary, scattered, and fluorescence photons interacting in a scoring volume were tallied at various depths from the source. Electron tracks were generated by sampling the photon …